Exterior light failure indicator

[Black Jack]

Hope this is not a rookie question but I could use help. I have checked every exterior light, turn signal, brake light, fog lights and they are all working. Failure light stays on all the time (when ignition is on). This is my 1st MB, a 560SL (1989). Owners manual states this light can come on if wrong bulb is installed. Is this my problem?

[Gilly]

It could be, the system is monitoring the current being consumed in each circuit, so if a bulb is way off (even cheap bulbs can be off enough) it can trigger the light.

Typically on an older one, the light doesn't come on until the circuit is on. If you start the car and the light is on, then it's pretty weird. Usually it takes, for example now, stepping on the brake or turning the light switch on before the warning lights up. This would be your clue as to where to look for problems.
Make sure you check the brake lights, get a friend to step on the brakes and check them out including the center stop light (I an sure yours has this on top of the decklid).

Couple areas that get overlooked by owners are the license plate bulbs and the small side marker bulbs on the rear tail lights (extreme left and right sides of the car, around to the side).

If you are positive everything works, then yes look for bulbs that simple have "CHINA" stamped in the bases or bulbs with very dark glass globes.

[SunGuy]

Originally Posted by Black Jack

Hope this is not a rookie question but I could use help. I have checked every exterior light, turn signal, brake light, fog lights and they are all working. Failure light stays on all the time (when ignition is on). This is my 1st MB, a 560SL (1989). Owners manual states this light can come on if wrong bulb is installed. Is this my problem?

I had this exact same problem and used Gilly's steps to try to find the cause of the failure indicator light. Most of the bulbs were Osram/Sylvania (OEM). If you need it, this link gets you to their bulb guide; type in your model info http://www.sylvania.com/en-us/applications/automotive-lighting-systems/Pages/lrgmain.aspx , and the system gives you the bulb #'s that should be in your car.

If yours check out OK as mine did, the next thing I checked out was the Lamp Control Unit (p/n 1075420132,(under the passenger floorboards next to the Engine Control Unit.) Small black box about the size of a pack of cigarettes. This unit reads the resistance on each of the circuits it monitors and if any are found to be out of spec, turns the failure light on. This part is where I finally found the problem. I installed a known good Control Unit from a friends car (takes all of 5 minutes) and the failure lamp went out. I replaced the unit with one from a nearby imported cars boneyard that only cost around $20. New out of the MB Dealer around $300
Good luck with yours-

Bob

[Black Jack]

Hey Guys, thanks alot for the info and suggestions. I'll take a closer look again at all the bulbs. If all checks out I'll try to find a Lamp control unit. I'll let you kmow as soon as I resolve the problem. Thanks again.